The government has rights in this invention pursuant to Air Force and Army Contract No. AF19628-80-C-0002.
Attention is directed to an article by the inventors and a colleague entitled "Passive Superconducting Microwave Circuits for 2-20 GHz Bandwidth Analog Signal Processing" Proc. of the IEEE Int'l. Microwave Symposium (June 15, 1982), hereby incorporated by reference.
The use of signals in 2-20 GHz range has become increasingly important in communications and radar. Consequently, construction of signal processing devices such as matched filters and linear phase filters, in particular, has become a goal for researchers.
At lower bandwidths various signal processing devices have been constructed that yield high time-bandwidth (TW) products. For example, analog discrete-time devices with bandwidths up to 20 MHz have been made with charge-coupled devices (CCDs); analog continuous-time devices with bandwidths up to 1000 MHz (1 GHz) have been made using surface-acoustic-wave (SAW) devices; and recent research effort has explored acoustooptic (A/O) devices and magnetostatic wave (MSW) devices, both with bandwidths of about 1 GHz. The propagating wave velocities in these devices are substantially below the speed of light; thus one can achieve large interaction times in relatively compact forms. However, a host of physical limitations such as propagation loss, dispersion, and transducer inefficiency prevents the practical utilization of these techniques at bandwidths above 2 GHz.
Elctromagnetic delay lines offer bandwidths of tens of gigahertz, well beyond those realizable with acoustic delay lines or sampled data structures such as CCDs. However, the high electromagnetic velocity requires the use of long lines to achieve significant delay. For example, a 100 ns device would require about 30 meters of free space delay or about 10 meters if the medium had a dielectric constant of 10. This length of coaxial cable or waveguide would be physically cumbersome. Such a delay also could be achieved with a copper microstrip delay line on low-loss 0.4-mm thick alumina substrate and would require an area of about 500 cm.sup.2. However, for 5-GHz bandwidth operation centered at 10 GHz, it would have a loss of about 40 dB at room temperature. Thicker substrates would give lower losses but would require larger area for a given delay. Because of this trade-off of large area or high loss, conventional electromagnetic delay technology has been unsuitable for microwave signal processing to date.
Nonetheless, there exists a need for signal processing devices in the 2-20 GHz range and electromagnetic delay lines constructed using microfabrication processing techniques would be particularly valuable as components in large scale integrated circuits. Specifically there exists a need for matched filters, in particular, "upchirp" and "downchirp" filters, and linear phase filters.